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A space elevator, commonly referred to alternatively as a "space tether", is the term given to an immense structure which is used to ferry large loads of materials into space. Space elevators generally consist of large structures of carbon nanofiber which span straight up from the ground, thousands of kilometers high, ending at stations in space. Vehicles using the structure derive their power from strands of superconducting material.[1] Space elevators are only known to have been constructed by humans since the 24th century.

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Space elevators are a common construction of the UNSC, both on Earth and her colonies. There were six on Earth, but only the New Mombasa Orbital Elevator in New Mombasa,[2] the Centennial Orbital Elevator in Havana,[3] and the Quito Space Tether in Ecuador[4] have been named. While Earth has six space elevators, many planets within the Outer Colonies have more since there is a heavy human reliance on the production and shipment of agricultural and mineral goods from remote worlds. Prior to the Covenant's invasion, the farming colony of Harvest, for example, had seven elevators linked to the orbital station Tiara, while some mineral-rich worlds had as many as nine.[5] During the events of the Human-Covenant war, many of these elevators were damaged, some seriously, during combat engagements. Of Earth's six elevators, only four remained intact after the Battle of Earth.[6]

A space elevator is a structure designed to transport and ferry different materials from a planet's surface into space and onto a platform. The base concept of a Space elevator consists of a cable attached to the surface on the equator and reaching outwards into space. By positioning it so that the total centrifugal force exceeds the total gravity, either by extending the cable or attaching a counterweight, the elevator stays in place in geosynchronous orbit. Once moved far enough, climbers are accelerated further by the planet's rotation.

The most common proposal is a tether, usually in the form of a cable or ribbon, that spans from the surface to a point beyond geosynchronous orbit. As the planet rotates, the inertia at the end of the tether counteracts gravity and keeps the tether taut. Vehicles can then climb the tether and escape the planet's gravity without the use of rockets. The engineering of such a structure requires an extremely light but extremely strong material (current estimates require a material ~2 g/cm³ in density and a tensile strength of ~70 GPa). Such a structure could eventually permit delivery of great quantities of cargo and people to orbit, and at costs only a fraction of those associated with current means with little of the danger of conventional sub-orbital travel.

The space elevator is gigantic, reaching thousands of kilometers in height. An orbital tether's center of gravity must be above or at a point of geosynchronous orbit above the body it is located on. Because geosynchronous orbit above Earth is quite high, (35,900 Km above the surface) the height of the elevator would be twice the distance from the surface to the point of geosynchronous orbit. This gives orbital tethers (Because the same rule would apply to them all) an average height of 70,000 kilometers above the Earth's surface.[7]

Space elevators vary in size and shape, but they are all typically composed of the same raw material. Meshed together as a complex composite of intertwining nanofibers, these ingredients form a series of massive cords and rings several hundred meters wide. They bind to a grounded set of Polycrete anchors larger than most buildings which hold the elevator's structure in place while the planet spins on its axis. The zenith of the elevator, commonly known as the "orbital" or "terminus" is then pulled taut by the planet's rotational inertia, sliding into geosynchronous orbit thousands of kilometers above the planet.[6]

The UNSC utilizes several designs of space elevators. Many space elevators, such as the New Mombasa Orbital Elevator, consist of a single tether reaching into space, surrounded by additional strands and massive support rings. The lower part of the tether is surrounded by an additional support frame. The Quito Space tether utilizes a similar design. Harvest's orbital elevator system, built about two hundred years later, consisted of seven separate strands of Carbon Nanofiber, attached to the orbital station Tiara. The climber system is often modular, and different types of containers, such as regular cargo containers, maintenance cars, or "Welcome Wagons" used to transport personnel can be used to move on the elevator strands. In high-capacity cargo elevator systems such as the Tiara, the cargo containers are also compatible with ground-based MagLev Train lines and they can be effectively converted into freighters by attaching a propulsion pod to them on the top of the tether.

The earliest concepts of space elevator date back to the end of the 19th century. During the second half of the 20th century and the early 21st century, multiple concepts were proposed, but it was not until almost three hundred years later that the first space elevator would be built. The construction of the first space elevator, the New Mombasa Orbital Elevator, begun in 2302. The cities with space elevators, "tether cities" as they came to be called, are often managed by second generation "dumb" AIs. As shown in New Mombasa, space elevators have a significant impact on the importance and economy of the cities they are located in. The cities and their surroundings are usually full of warehouses, to store the massive amounts of cargo transported to and from orbit. During 2552, Earth had six tether cities, each managed with the help of an AI.[8]

Due to the size of the space elevator, the safety of such a structure is an obvious concern. The catastrophic effects of a space elevator's collapse were witnessed multiple times during the Human-Covenant war, when many of UNSC's space elevators collapsed due to the fighting.

If the orbital counterweight is destroyed or the tether is cut near the top, the whole cable will fall, usually wrapping itself around the planet; the New Mombasa Orbital Elevator would be able to wrap itself around the Earth at least two times. This happened to Harvest's elevator system when Loki destroyed the Tiara station with a Mass Driver. The Centennial Orbital Elevator on Earth also collapsed due to the destruction of Station Wayward Rest at its top.

If the tether is cut halfway up, the upper portion will rise up and remain in orbit while the lower part will drape around the planet. The same will occur if the tether breaks a quarter way up. In case the break occurs at or near the anchor point on the planet surface, the whole tether will rise upward and end up in an unstable orbit around the planet. This was the case when New Mombasa's orbital elevator collapsed due to the damage caused to it by a Slipspace rupture backlash.

Due to the safety concerns discussed above, tether cities are always designed with the possibility of a catastrophe in mind. The cities are often compartmentalized into multiple symmetrical sections, to minimize the death toll and property damage in case anything were to happen to the elevator.[9] However, contrary to the popular belief that the collapse of a space elevator will cause massive planetary destruction, the tether itself will not cause any significant damage at all. The lightweight construction of the tether allows for air resistance to negate the effects of gravity. Instead, it will be the support structure that surrounds the tether that causes most of the damage, as evidenced by the wreckage strewn across the Tsavo Highway.